There's also, I suspect some disinterest in retreading a story that they know so well, similar to what some folks felt about a High School Peter Parker pre-Homecoming. If you fall in love with a mature version of the character, then seeing them immature feels like personally losing something, and some people still have a sore spot with his sidekicking to Tony.
Yes them being young and starting out when most contemporary heroes are also pretty new to their roles is almost entirely different to them starting out when other heroes have already been saving the world and getting adored and/or despised by the public for years and actually over a decade. Let alone if/when you have (as probably will happen) them outright looking up to the older established heroes.
Another significant issue with starting them and having them be real young (teenagers/HS) is that the actors (even newcomers) probably won't want to do more than a few films so if you have two or three solo HS years films and three crossover appearances you wouldn't ever get the adult experienced versions (or in just one film) short of recastings. That would be a lot less of a problem if the MCU started having a lot fewer crossover appearances but that's unlikely.
The Harry Potter films were an inspiration for Marvel Studios Spider-Man and may be for its X-Men but actors being willing to do a Harry Potter-length series/character portrayal seems unlikely.


